Heating devices used for the above purposes are disclosed in Japanese Patent Application Laid-open No. 2-59356 and in Japanese Patent Application Laid-open No. 2-65086 for example. Such a heating device includes a strip-like heating resistor layer formed on a substrate made of a heat-resistant insulating material such as ceramic for example, and a protective layer formed on the substrate to cover the heating resistor layer. Typically, the protective layer is made of a glass material and arranged to withstand the heat generated at the heating resistor layer while insuring electrical insulation from the exterior and avoiding wear due to contacting with a sheet material fed relative to the heating device.
In such a heating device, it is necessary to insure a sufficient electrical insulation, since a considerably large current is passed through the heating resistor layer to generate Joule heat for heating the sheet material. However, generally, a conventional glass material used for the protective layer has a dielectric strength of only about 14-15 volts per thickness of 1 .mu.m. Thus, it is necessary to make the thickness of the protective layer considerably large for insuring a sufficient electric insulation. As a result, in the conventional heating device, the heat capacity of the protective layer becomes large, so that the thermal response at the surface of the protective layer is likely to deteriorate (the temperature rises slowly). If, to compensate for this, the amount of the heat generated at the heating resistor is increased, a problem of wasting energy will occur due to low thermal efficiency.